DESCRIPTION

The function
posix_memalign()
allocates
size
bytes and places the address of the allocated memory in
*memptr.
The address of the allocated memory will be a multiple of
alignment,
which must be a power of two and a multiple of
sizeof(void *).
If
size
is 0, then
the value placed in
*memptr
is either NULL,
or a unique pointer value that can later be successfully passed to
free(3).

The obsolete function
memalign()
allocates
size
bytes and returns a pointer to the allocated memory.
The memory address will be a multiple of
alignment,
which must be a power of two.

The function
aligned_alloc()
is the same as
memalign(),
except for the added restriction that
size
should be a multiple of
alignment.

The obsolete function
valloc()
allocates
size
bytes and returns a pointer to the allocated memory.
The memory address will be a multiple of the page size.
It is equivalent to
memalign(sysconf(_SC_PAGESIZE),size).

The obsolete function
pvalloc()
is similar to
valloc(),
but rounds the size of the allocation up to
the next multiple of the system page size.

For all of these functions, the memory is not zeroed.

RETURN VALUE

aligned_alloc(),
memalign(),
valloc(),
and
pvalloc()
return a pointer to the allocated memory, or NULL if the request fails.

posix_memalign()
returns zero on success, or one of the error values listed in the
next section on failure.
The value of
errno
is indeterminate after a call to
posix_memalign().

ERRORS

EINVAL

The
alignment
argument was not a power of two, or was not a multiple of
sizeof(void *).

ENOMEM

There was insufficient memory to fulfill the allocation request.

VERSIONS

The functions
memalign(),
valloc(),
and
pvalloc()
have been available in all Linux libc libraries.

The function
aligned_alloc()
was added to glibc in version 2.16.

The function
posix_memalign()
is available since glibc 2.1.91.

CONFORMING TO

The function
valloc()
appeared in 3.0BSD.
It is documented as being obsolete in 4.3BSD,
and as legacy in SUSv2.
It does not appear in POSIX.1-2001.

Headers

According to SUSv2,
valloc()
is declared in <stdlib.h>.
Libc4,5 and glibc declare it in <malloc.h>, and also in
<stdlib.h>
if suitable feature test macros are defined (see above).

NOTES

On many systems there are alignment restrictions, for example, on buffers
used for direct block device I/O.
POSIX specifies the
pathconf(path,_PC_REC_XFER_ALIGN)
call that tells what alignment is needed.
Now one can use
posix_memalign()
to satisfy this requirement.

posix_memalign()
verifies that
alignment
matches the requirements detailed above.
memalign()
may not check that the
alignment
argument is correct.

POSIX requires that memory obtained from
posix_memalign()
can be freed using
free(3).
Some systems provide no way to reclaim memory allocated with
memalign()
or
valloc()
(because one can pass to
free(3)
only a pointer obtained from
malloc(3),
while, for example,
memalign()
would call
malloc(3)
and then align the obtained value).
The glibc implementation
allows memory obtained from any of these functions to be
reclaimed with
free(3).

The glibc
malloc(3)
always returns 8-byte aligned memory addresses, so these functions are
needed only if you require larger alignment values.

SEE ALSO

COLOPHON

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man-pages
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and the latest version of this page,
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